In the spring of 1790, he left Göttingen, and, with Foster and Van Genns, took a journey to the Lower Rhine, Holland, Belgium, England, and France, studying docks, mines, botanic gardens, manufactures, and churches, and visiting literary celebrities. Still the new friends did not take the place of the old, for he writes to Wegener: "I beseech you, dearest Wegener, by all the affection which you know I bear you, never to forget our brotherly love and friendship. You are infinitely more to me than I can ever be to you. I have now seen the most celebrated places in Germany, Holland, and England--but, believe me, I have in seeing them never been so happy as while sitting in Steinbart's arm-chair." The influence of this journey was never lost. Sixty-eight years afterward, Humboldt said: "For the space of thirty years I have never known leisure but of an evening, and the half-century that I have spent in this ceaseless activity has been occupied in telling myself and others how much I owe my teacher and friend George Foster in the generalization of my views on nature, and in the strengthening and development of that which had already dawned in me, before those happy days of intimate friendship." In the latter part of 1790, Humboldt went to Hamburg, to enter the School of Commerce. He wished to study political economy further, and to learn practical book-keeping. He wrote to a friend: "I am contented with my mode of life at Hamburg, but not happy, less happy even than at Göttingen, where the monotony of my existence was relieved by the society of one or two friends and the vicinity of some moss-grown mountains. I am, however, always contented when I feel that I am accomplishing the purpose I have in view.... My leisure hours are occupied with geology and botany.... In addition, I have begun to learn Danish and Swedish." To Wegener he writes: "I have made considerable progress in general information, and I am beginning to be somewhat more satisfied with my attainments. I worked very hard at Göttingen, but all I have learned makes me feel only the more keenly how much remains still to know. My health suffered severely, but improved somewhat during my journey with Foster; yet even here I continue so closely occupied that I find it difficult to spare myself. There is an eager impulse within me, which often carries me, I fear, beyond the bounds of reason; and yet such impetuosity is always necessary to insure success." The "eager impulse" was a sure indication of something to be accomplished by and by. Success does not come with half-hearted effort; it comes only through a force and persistence that will allow no barriers between us and the goal.